Variables

R uses <- for variable assignment.

Don't call your variables any of the following:

c, q, s, t, C, D, F, I, T

diff, length, mean, pi, range, rank, time, tree, var

if, function, NaN etc.

Howard Seltman provides more information about reserved terms in this "Learning R" lesson.
You can use underscores and periods in your identifiers. Google suggests tree.count for variables and DoSomething for functions.